Fig. 1. The crystal structure and corresponding simulated diffraction patterns of η-MgZn2 viewed from different zone axes. Green and orange spheres represent Zn and Mg atoms, respectively.

Fig. 2. DSC curves of the as-quenched alloy 7075 under the scanning rates of 3, 10 and 30 K/min. Inset: Enlarged image of DSC Peak I and Peak II.

Fig. 3. HAADF images of phases formed in AA 7075 heated to (a, b) 381 K at 3 K/min, (c, d) 418 K at 10 K/min and (e, f) 459 K at 30 K/min with FFT patterns inlay, corresponding to the end of Peak I + II in the DSC scans. Images are taken from the [100]Al zone axis.

Fig. 4. HAADF images of phases formed in AA 7075 under different heating rates, which are the same as Fig. 3 but taken from the [110]Al zone axis.

Fig. 5. Low-magnification HAADF images and representative atomic scale images with associated FFT patterns of phases formed in samples heated (a-c) at 3 K/min to the end of Peak III in the DSC scans (459 K) and (d-f) at 10 K/min to the lowest point between Peak III and Peak IV (488 K).

Fig. 6. HAADF images and corresponding FFT patterns for another two precipitates formed in samples heated (a, b) at 3 K/min to 459 K and (c, d) at 10 K/min to 488 K. Red dashed lines mark out the RR-1 stackings existing in the precipitates. Light blue dashed lines in (c) reveal a similar stacking between the upper part of the precipitate and the Al matrix. Insets in (a) and (c) are the processed enlarged images of the areas in the white dashed boxes. Green and orange dashed circles represent Zn and Mg atoms, respectively.

Fig. 7. HAADF images and associated FFT patterns of two η1' precipitates formed after heating (a, b) at 3 K/min to 459 K and (c, d) at 10 K/min to 488 K. Light blue dashed lines in (a) and (c) indicate the similar structure between the η1' and the Al matrix. Insets in (a) and (c) are the enlarged images of the areas indicated by the white boxes. Zn atoms are located based on their strong bright contrast and marked out by green dashed circles. Red dashed lines represent the incomplete RR-1 stackings with Zn atoms as the rhomb corners. White arrows point at the occasionally existing atoms on the rhomb edges. In the FFT patterns, the spots associated with the precipitates and Al matrix are identified and the d-spacings of the specific diffraction spots of the precipitates are measured and calculated.

Fig. 8. HAADF image and EDS elemental mappings of one η1' precipitate.

Fig. 9. (a) Schematic illustration of unit cells, the η1, the proposed η1' and a supercell of η1' showing the periodically missing edge Zn atoms in the R units. Dashed circles indicate vacant positions. (b) The enlarged HAADF image of the η1' in Inset 2 of Fig. 6(c) together with the superimposed atomic structure of the proposed η1' model and Al matrix. The white arrow points at the deviation of the proposed atomic model from the HAADF image. (c) The same HAADF image with the simulated image obtained from an Al-precipitate-Al sandwich structure [48]. The atomic position supposed to be occupied (solid white circle) but vacant in the simulated image (dashed white circle) is pointed out.

Fig. 10. Schematic of the transformation process: solid solution → η1' → η1.

Fig. 11. Low-magnification HAADF images of precipitates formed in samples heated to the end of Peak IV in the DSC scans (a) at 3 K/min to 501 K; (b) at 10 K/min to 523 K; (c) at 30 K/min to 559 K.

Fig. 12. Representative atomic-scale HAADF images and corresponding FFT patterns of precipitates formed in samples heated to the end of Peak IV in the DSC scans at 3 K/min to 501 K (a, b, d and e), 10 K/min to 523 K (h-k) and at 30 K/min to 559 K (l-o). In the FFT patterns, the spots associated to the precipitates and Al matrix are identified and the d-spacings of the specific diffraction spots of the precipitates are measured and calculated. (c) The inverse FFT of the region marked with a white dashed box in (a) and the “⊥” indicates the dislocation. (f, g) The enlarged images of the areas indicated by the white dashed boxes in (d). (f) Some flattened hexagonal units (marked with green) surrounded by R units (marked with red). In (g), Zn atoms are located based on their strong bright contrast and marked out by green dashed circles. Red dashed lines represent the incomplete RR-1 stackings with Zn atoms as the rhomb corners.

Fig. 13. Evolution of the average cross section (a) and number density (b) of phases formed in different aging conditions. The lines are guidelines only.